1. Field of the Invention
The present invention relates to a driving method of a variable reistance element including a perovskite-type oxide between a first electrode and a second electrode, of which electric resistance is changed by applying a voltage pulse of a predetermined polarity between the both electrodes, and a memory device provided with the variable resistance element.
2. Description of the Related Art
In recent years, as a next-generation Nonvolatile Random Access Memory (NVRAM) capable of being operated at a high speed in place of a flash memory, various device structures such as a Ferroelectric RAM (FeRAM), a Magnetic RAM (MRAM), and an Ovonic Unified Memory (OUM) or the like are proposed and in view of a high performance, a high reliability, a low cost, and a process matching, competition in development of them has been intensified. However, these current memory devices have both merits and demerits respectively and it is difficult to realize the ideal “universal memory” having each merit of a SRAM, a DRAM, and the flash memory.
On the contrary to these existing arts, a method of changing an electric resistance in a reversible fashion by applying a voltage pulse to a perovskite material that is known as a supergiant magnetic resistance effect is disclosed by Shangquing Liu and Alex Ignatiev et al. of University of Houston (refer to the specification of U.S. Pat. No. 6,204,139, Liu, S. Q. et al., “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films”, Applied Physics Letter, Vol., 76, pp. 2749-2751, 2000). This is very epoch-making because the resistance change across several digits appears even at room temperature without applying a magnetic field while using the perovskite material that is known as the supergiant magnetic resistance effect. A Resistive Random Access Memory (RRAM) does not require any magnetic field differently from the MRAM, so that this has an excellent advantage such that the power consumption is very low, minuteness and high integration can be easily realized, and multilevel storage is possible since a dynamic range of the resistance change is very large as compared to the MRAM. The basic structure of the real device is very simple, and it is made in such a manner that a lower electrode material, the perovskite-type oxide, and an upper electrode material are deposited in a direction vertical to a substrate. In the meantime, according to the element structure illustrated in the specification of U.S. Pat. No. 6,204,139, the lower electrode material is made of a yttrium barium copper oxide YBa2Cu3O7 (YBCO) film deposited on a single crystal substrate of a lantern—aluminum oxide LaAlO3 (LA); the perovskite-type oxide is made of a crystalline praseodymium calcium manganic oxide Pr1−xCaxMnO3 (PCMO) film; and the upper electrode material is made of an Ag film deposited by sputtering, respectively. It is informed that the operation of this memory element can change the resistance in the reversible fashion by applying 51 volt of the voltage pulse between the upper and lower electrodes in positive and in negative. It means that a new nonvolatile memory device is available by reading a resistance value in this reversible resistance change operation (hereinafter, referred to as “the switching operation” appropriately).
However, the present inventor variously conducted a study regarding the nonvolatile memory device by using the perovskite-type oxide of which resistance is changed due to the voltage pulse, particularly, the PCMO film, and as a result, there are problems such that the switching operation is not always carried out, for example, the switching operation is not carried out although the resistance is changed due to the voltage pulse and the resistance value is not changed. These problems are major barriers to practical application thereof. In other words, it means that the nonvolatile memory device that can be repeatedly rewritten cannot be realized if the stable switching operation cannot be secured.